For how many fake registrations are we talking about? I doubt they are real humans if many.
Is that a big and popular forum?
Having a security protection plugin goes beyond fake registrations or spam.
quite a big forum yes, see attached
i dont like using security plugins, most slow the site down... the server has good security... what security plugin do you recommend?
Previously i suggested a security plugin for your issue, because some plugins also check IPs against a database (cloud) and deny access. And because spammers (bots or human) are from KNOWN Ips, they will be blocked.
Spam and profiles are imho best to be blocked/prevented at login/registration and not AFTER.
There was a similar problem. They helped me solve it here. And in addition to captcha wpforo, I installed the Captcha by BestWebSoft plugin. Since then I have not seen a single spambot.